Changes in sleep-wakefulness were studied in male Wistar rats after destruction of the medial septal neurons with NMDA. Electroencephalogram, electromyogram and electrooculogram were recorded for 24 h prior to the destruction of the medial septum, and 7, 14 and 21 days after the destruction. There was a decrease in the total amount of slow wave sleep and frequency of slow wave sleep episodes after the lesion. It also produced an increase in the duration of paradoxical sleep episodes. These findings are in contrast to the changes produced after lesion of other basal forebrain areas. The present findings suggest that the medial septum may be involved in the genesis of slow wave sleep and inhibition of the durations of paradoxical sleep episodes.